HIGHER EDUCATION: It is Einstein versus Newton again
ON Nov 10, 1919, the New York Times carried a headline, "Lights all askew in the heavens/Men of science more or less agog over results of eclipse observations/Einstein theory triumphs" that turned the scientist world famous overnight.
The headline referred to the solar eclipse expedition, led by the British astronomer Arthur Eddington, which confirmed the prediction by Einstein using his theory of gravity -- general relativity -- for the bending of light from distant stars by the sun's gravity.
Sir Isaac Newton's three-century-old theory of gravity, although overthrown by Einstein's theory, is still believed to be a good approximation to the latter's theory if gravity is weak.
Moreover, the general relativistic prediction for the motion of a particle influenced by gravity is believed to be well approximated by the Newtonian prediction if the particle speed is low compared to the speed of light and gravity is weak.
However, in a paper published in the journal PLoS ONE on April 19, researchers Associate Professor Lan Boon Leong and Liang Shiuan-Ni of the School of Science, Monash University Sunway campus, showed with a simple bouncing ball system that the predictions of the two theories do not always agree as conventionally expected.
They showed that the two predictions for the ball's trajectory can rapidly diverge and become completely different if the motion of the ball is chaotic.
When the predictions contradict each other, who is right: Einstein or Newton?
The bouncing ball system studied by Lan and Liang can be realised in the laboratory -- for example, as a steel ball bouncing on a concave lens which is attached to the oscillating membrane of a loudspeaker. In between impacts with the lens, the ball undergoes free-fall motion due to gravity if the set-up is housed in a vacuum chamber.
Lan explains: "However, it is very difficult to accurately calculate the ball's trajectory using the two theories for comparison with experiment because the parameters and initial conditions of the system must be known precisely when the motion is chaotic."
He adds: "But, since general relativity has withstood other experimental tests over the century, one would expect the general relativistic prediction to be empirically correct. This would mean that even for low-speed weak-gravity systems, general-relativistic mechanics must generally be used, instead of the standard practice of using Newtonian mechanics, to correctly study their dynamics."
This paradigm shift may well lead to new understandings and discoveries for such systems.
"On the other hand, if the Newtonian prediction is correct, Einstein would surely turn in his grave -- his theory would need to be corrected in the low-speed weak-gravity limit.
"Either way, there is new Physics to be explored."
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